UPWARD COLLAPSE PROCESS AND APPARATUS FOR MAKING GLASS PREFORMS

An apparatus and related process for producing large glass preforms with minimal clad to-core waveguide distortion from a glass body having a weight, an outer surface, core rods, and a cladding surrounding and separated from the core rods by a gap. The apparatus includes collars affixed to the top and bottom of the cladding; a first unit holding and supporting the bottom collar; a second unit holding and supporting the top collar; and a frame defining a heating zone having a heating element to heat the glass body, collapse the cladding onto the core rods, close the gap, and create the glass preform. The weight of the glass body above and below the molten glass in the heating zone is supported by the first and second units without contacting the outer surface of the glass body, wherein the bottom collar holder and vacuum unit is adapted to be moved at a precisely controlled velocity V1 in the vertical direction and the top collar holder and vacuum unit is adapted to be moved at a precisely controlled velocity V2 in the vertical direction, such that when a vacuum is applied from either the bottom collar holder and vacuum unit or the top collar holder and vacuum unit, the top collar, the cladding and the core rods can be collapsed to form the glass preform.